Align factor/texture spec language with intended usage

_Originally mentioned in https://github.com/KhronosGroup/glTF/discussions/2443._

The spec shouldn't say "If both factors and textures are present" when talking about the base color factor value being linear.

Start by locating the glTF specification text about the base color factor being linear and the phrase "If both factors and textures are present." Update the specification wording to match the intended usage described in this issue, then verify that the conditional language no longer misstates when the factor value is linear.

Written by the indexing model from the issue text.